By now, the following existing techniques have been searched: 1. CN200510127583.5, entitled “Pressure Control Systems for Automatic Transmissions”; 2. CN200510117492.3, entitled “Hydraulic Control Systems for Automatic Transmission Units for Vehicles”; 3. U.S. Pat. No. 6,591,958, entitled “Pressure Control Apparatus for a Torque-transmitting Mechanism”; and 4. CN200510084452.3, entitled “Regulating valves of torque-transmitting mechanisms and method for engaging torque-transmitting mechanisms”.
In an automatic transmission, the transmission torque of the torque control devices such as clutches, brakes or brake bands mainly depends on a jointing pressure of a piston chamber thereof. During the jointing process of the torque control devices such as clutches, the transmission torque thereof is controlled by controlling the jointing pressure. A TCU (short for automatic Transmission Control Unit, alternatively translated as Electronic Control Device) outputs a signal to control an electromagnetic valve, and a pressure of the electromagnetic valve controls a pressure regulating valve to provide this jointing pressure, wherein the pressure of the electromagnetic valve serves as a pilot control valve. Thus, if it is desired to ensure an adequate torque capacity, the resolution of a pressure control will reduce, which is disadvantageous for the precise control to a torque device.